It is well known in the literature that the presence of naphthalene moieties or residues (typically derived from a NDA or diester monomer) in polyesters imparts increased barrier performance and thermo-oxidative stability. Both of these attributes are important for polyesters which are used in packaging various beverages including water, carbonated soft drinks, and beer. In addition, naphthalene residues are particularly useful in certain polyesters such a liquid crystalline polymers due to the formation of a sideways displacement in the polymer backbone that imparts processability to high temperature polymers including liquid crystalline polymers without a significant reduction in the level of three-dimensional crystallinity. High levels of crystallinity result in higher heat distortion temperatures that are essential in many applications such as heat stable containers, electrical connectors, automotive applications, and computer hardware.
Although polyesters derived from NDA such a poly(alkylene naphthalenedicarboxylate) have been known in the literature for many years, the commercial availability of such a polyester, poly(ethylene 2,6-naphthalenedicarboxylate) (PEN) is a relatively recent event. PEN possesses a higher glass transition temperature (T.sub.g), e.g., 125.degree. C., compared to the T.sub.g poly(ethylene terephthalate) (PET), e.g. 80.degree. C. In addition, it is well known that the barrier to oxygen exhibited by PEN is five times greater than that exhibited by PET. As a result, it is expected that PEN will experience increased use in beverage container applications and high performance film applications. Due to the desirability of plastic recycling, the conversion of used or waste PEN polyester polymer to useful materials, especially 2,6-NDA, is a particularly important and desirable objective.
Processes for the recovery of naphthalene moieties or residues from poly(alkylene naphthalenedicarboxylates) polyesters are known. For example, Published PCT Patent Application WO 8911471 describes the formation and recovery of dialkyl 2,6-naphthalenedicarboxylates from poly (alkylene 2,6-naphthalenedicarboxylates) polyester polymers. The process described involves the use of an alcoholic solvent in the presence of a transesterification catalyst. U.S. Pat. No. 5,414,113 describes the recovery and purification of aromatic dicarboxylic acids from waste polyester film, fiber, bottles, and manufacturing residues. The process comprises the depolymerization of polyester resin in a solvent followed by an oxidative purification step. U.S. Pat. No. 5,473,102 discloses a process comprising depolymerization of a polyester resin in a molten polyester resin containing solvent with superheated steam followed by high temperature distillation of the dicarboxylic acid monomer for purification.